If it's going to be a dark dreamkeeper, then I would bet on Tinsil, just because I could see her playing sides to her own advantage that way.
I also thought it odd, and a little too unconvincing, when after being called a "Traitor" by Nabonidus, she got upset, and the he told her that he was angry about wisp and she calmed down instantly and got playful.
At first, I thought she switched geers like that because he said "If not for the return of Void, I would smite you where you stand." It seemed to fit back then, but I kept coming back and never quite believed it.
Then this thread started and I began to wonder if there was a connection Void, Scuttler and her.
Nabonidus was too angry to wonder and it took me years to think it was posible that she even could!

Nah, Tinsel is ruthlessly ambitious and loyal to power only...Nabonidus would be very foolish if he wasn't dangling a big enough carrot (real or not) in front of her. Not doing so would of course be an invitation for treachery if Tinsel was convinced Void (or any other Nightmare) could do better by her. That being said, she's no idiot either so she'd probably have some understanding that Void could very well kill her or worse, spoil her ambitions if Nabonidus was overthrown.

I cancel Ravat out too, because in my head at least, I think he'd have just tortured and killed the twins on the spot rather than getting involved in any elaborate plans. To me, causing anarchy and chaos (amongst Nightmares or DreamKeepers alike) doesn't seem something he'd have any interest in, unless he benefited from having more playthings to kill and torture.